Biological processes of maturation appear to play an important role in the development of patterns of differential abilities. Because girls mature earlier than boys physically, any behavioral differences which correlate with rate of physical maturation will also correlate with sex. In previous work, we demonstrated that rate of physical maturation is systematically associated with certain mental abilities and thus provides a new explanation for typically reported sex differences in these abilities. In order to determine whether other aspects of cognitive and social functioning are influenced by maturational rate, we are presently studying other behaviors known to be related to sex in a manner similar to the one used in the previous study. Specifically, we are studying, prior to the onset of puberty and at puberty, rates of both physical and neuromotor development as they relate to (a) differential abilities; (b) cognitive processes involved in the performance of tasks shown to be related to maturational rate; (c) variation in hemispheric organization of function; and (d) interpersonal behaviors which have been demonstrated to be related to these cognitive abilities and to sex. The data are being collected in a longitudinal design. An initial sampling of 90 pre-pubertal boys and 90 pre-pubertal girls from a middle-class community was evaluated. These children will be retested two years after the initial testing, when most of them would be expected to show at least initial signs of puberty, thus making it possible to discriminate between early and late maturing individuals. If we show that these behaviors are related to maturational rate independent of sex, then we will have provided evidence that biological mechanisms contribute to their development. In addition, the study can provide data which will be useful in the investigation of specific mechanisms by which biology influences behavioral development.